Patients with chronic hepatitis B and treated with entecavir for over 6 months (with no previous other antiviral treatment) will be invited to participate in this study. They will be eligible if their liver tests are normal and their viral load is undetectable. Patients will be switched to lamivudine treatment to assess whether lamivudine can maintain adequate suppression of the hepatitis B virus after successful treatment with entecavir. Patients will be monitored closely after switching treatment at 1 months and then every 3 monthly. If there is any evidence of increase in viral load then patients will be given the option of changing back to entecavir.